Information storage devices



April 24, 1956 M. L. scHuL-rz ETAL 2,743,430

INFORMATION STORAGE DEVICES Filed March 1, 1952 Paz/iwf 5MM/6 bfi Dir/cf- AVC,

:inventors 'MELVIN I.. B :HuL-rz EEEJREE H MDRTDN Gttomeg INFORMATION sroRAGE DEVICES Melvin L. Schultz and George A. Morton, Princeton, N. J.,

assignors to Radio Corporation of America, a corporation of Delaware Application March 1, 1952, Serial No. 274,485

16 Claims.` (Cl. 340-473) This invention relates to devices for storage of information. f More particularly, the invention relates to improvements in devices which store information in response Fto alight stimulus and release the stored information, also, in response to stimulation of light energy.

The invention is based upon the discovery that a certain light-sensitive material can be made photo-voltaic .witli a'polarity in one sense if current is first passed through in a lcertain direction, while itv can be made photo-voltaic with a polarity in the opposite sense if cur- 'rent is lirst passed through it in the opp'osite direction. When a photo-voltaic element of this nature is placed in electrically conductive contact with another element composed of photo-conductive material, which acts as a light switch,'apparatus may be constructed for storing information which may exist in either of two different forms and for making this information available at a later time. The storage effect decreases with time, however, the lim'it'of storage length being about l hour after activation has occurred. An activated lm of lead sulfide is preferred as the directionally sensitive photo-voltaic 'material, although" the selenide or telluride of lead may also be used. The photo-conductive material may be cadmium sulfide, selenide, or telluride, selenium, antimonyitrioxide or trisultide, mercuric sulde, or any one of many' other materials well known yto vhave this property.

Lead sulfide has previously been known as a photo- 'conductive material sensitive to ordinary light energy and, when specially activated, has also been known to be sensitive to energy in the infra-red portion of theenergy spectrum. The present invention, ho'wever, is based upon the discovery of another particular novel property of an activated film of lead sulfide. The lead sulfide may be deposited'from the vapor phase in vacuo and then activated by heating in the presence of oxygen for about to minutes at v1100"-475 C. lf a current is'then passed through it in one direction, it has now been found that the activated film is modified by electrolytic migration so that ifordinary light energy is directed upon it, it will exhibit a photo-voltaic effect with a polarity in one sense. If the current'is passed through in the opposite direction -and light is again directed upon the lm, it will then exhibit a photo-voltaic effect with polarity in a sense opposite to that exhibited previously.

. One object of the present invention is to provide a novel light-sensitive information storage cell.

,Another object of the invention is to provide ran improved apparatus for. storing information of a binary nature.

Another objectpof the invention is to provide an improved photo-sensitive memory device.

A further object of the invention is to provide an irnproved apparatus for recording and, later, delivering information.

A still further object of the invention is to provide an improved.lightserisitive memory cell which is simple to construct and operate.

These and other objects will be more apparent and the invention will be morereadily understood from the following'description, including the drawings, of which:

- Figure l is a plan view of a novel unit cell constructed in accordance with the present invention,

Figure 2 is a diagrammatic view of apparatus, including the cell of Figure l in cross section, for recording information and for later reading the stored information,

Figure 3 is a screen containing a plurality of unit cells such as illustrated in'Figure l, and

Figure 4 is an apparatus utilizing the screen of Figure 3.

Referring now to Figure l, there is shown one embodiment of a unit cell constructed in accordance with the invention. The cell may comprise an insulating base plate 2, which may be of glass, mica, or other material having electrical insulating and heat resistant properties. One of the major surfaces of the base plate is provided with two oppositely positioned electrodes 4 and 6. These may be prepared by painting a stripe of colloidal graphite suspension along two opposing edges of the surface and then drying. These electrodes may also be of gold or platinum or some other conductive material which does not react with the surface coatings later to be applied.

Upon one-half the surface of the plate, including that part covered by one of the electrodes 4, there is then deposited a coating of lead sulfide 8. This may be deposited by masking the portion of the surface not to be covered, placing the base plate in a vacuum chamber and then evaporating and depositing the lead sulfide. The lead sulde may be evaporated from a tantaluni boat heated with a tungsten filament and the pressure in -the vacuum chamber may be of the order of 10-5 mm. of mercury. The thickness of the deposited film is' not critical. The deposited film is then activated by heating it in the presence of oxygen for about 5 to 20 minutes at a temperature of about 400-475 C.

The next step of preparing a unit cell is the deposition of the photo-conductive portion 10. This may be done by`masking the coating S already deposited, placing the partially coated base plate once more in the'vacuum chamber and depositing a coating of a photo-conductive material on the unmasked portion of the plate such that the photo-conductive material is in contact with the activated'lead'sullide. The deposition may be carried out in a manner similar to the deposition of the lead sulfide previously described. The material is then given such activation treatment as may be required by the particular photo-conductor used. As also indicated previously, the photo-conductive portion may comprise a coating of cadmium sulfide or any one of many other materials.

The cell describedabove may be utilized in information storage apparatus, one simple example of which is illustrated in Figure 2. The electrodes 4 and 6 of the cell of Figure l are connected through leads 12 and 14, respectively, to the poles of a' conventional reversing switch 16, which may be either solenoid operated or -manually operated. A source of direct current, such as a battery 18, is connected to one side of the switch. There mayalso be provided alight source 20'having a switch 22 for turning the source oi and on as desired. An optical system 24 may also be provided for focusing the light from the source onto the coated surface ofthe cell.

To store information on the cell, the light source is turned on and the cell illuminated, causing the photoconductive portion 10 to assume a conductive condition. When used as a' binary memory ydevice in which the information may be Asuch as to indicate either one of two dilerent conditions (for example, to indicate l or 0),

current may be passed through the cell either from a to b Vmainlead 38. :upon or read from each element, -using any'well `known -system such .as that illustrated in Figure 4.

3 or from b --to atto establish the reading polarity. The information 1 .may lcorrespond to the .condition existing when a is and b is The information 0 will then correspond to the condition existing when a is and b is L+. 'If it iis then desired to store @the information 1, the switch =16 Ais 'controlled such that current -is made to flow from a to Jb. `If it is desired to store the infomation 0, the switch is reversed such that current is caused to flow from b to a. In order to control the .direction of current flow, the yreversing switch 16 may either Abe controlled manually by yan operator or Vautomatically from a source such as a punched tape.

The current may be passed through the cell for a ile'ngthof .time from the order of a few :hundredths .of a

second to several seconds; i. e.,.4.o'r V5. 'lhevoltagexof the current source Vmay be from la-.few volts, say, about -10, -to yseveral '-hundred. Although v.neither -time or .passage Vof. the current nor magnitude .of voltage is critical, the

subsequentoutputof fphoto current will depend upon the magnitude of these two factors. The'longer the time of passage of current and the higher the voltage the higher i -thezoutput current up to saturation.

To read back the stored information, thecurrent from .'th'esource 18 is cut off and the coated surface ofthe cell, including the pho'to-voltaic portion y8 is again illuminated bythe light source 20. This activates both the photoconductive and the photo-voltaic portion and a small -current will be generated inthe electrode .leads, the electrodes having a polarity depending upon the vmanner in which current was originally passed through the cell when the information was stored. To detect the polarity, any polarity-sensing ydevice 24, such ,as a galvanometer or ahigh impedance vamplifier is lconnectedvto `the electrode leads of the cell. This mayihave visible indicating means, such as lamps 26 'and';28, one .of which lights when the-voltage is of one polarity and the other of which lights Awhen the voltage is of. opposite polarity. For example, ifa galvanometer'is used, deflection -of the armature in one direction can be utilized to close con- :tacts to one of the lamps, while opposite deflection of the :armature can be used to close another pair loft contacts closing a circuit through the other lamp. The information ymay Aalso be recorded on a photographic, magnetic, or lpunched tape.

,In order to store a large number of items of information,

a plurality of storage elements, such as described 'in connection with Figure l, may be incorporated on a screen 30 such as illustrated in Figure 3. This screen comprises .a relatively large base plate 32, which maybe of fglass, '.with a large number of cells 34 on one Vof its surfaces. Each cell has electrodes similar 'tovthose described in connection with the single cell. All corresponding electrodes on the side of each cell containing the photo-voltaic material 'are connected in parallel to one of themain leads 36,

while all the electrodes in contact with the photo-conductiveinaterial portion of each cell are connected-,to -another Information can thenbe stored rapidly Referring now to Figure 4, a multi-element screen 30 is'placed, preferably with its coated'face adjacent lthe phosphor coated screen 40 of a cathode ray-tube 42 hav- 'ing'horizontal deflection yoke transformer coils 44 and lvertical-yoke transformer coils v46. rIfhe phosphor'ernf ployed in this type of tube is of the short-decay lperiod type. Each of the deflection `yokes lis lprovided with a Vsource f A.C. potentialfor sweep voltage purposes. As the electron beam is swept across the face of the tube, ,the spotof light illuminates each unit cell in turn and as eachvcell is illuminated, the reversing switch (as shown in Figure 2) is `operated and D.C. potential is applied to the leads 48 and 50 such'that current flows throughy the illuminatedcell in either one direction lor the other. This operation .conditions each cell so -that when itis later 4 illuminated to read the information it contains, it will vexhibit a polarityeither in ,one sense or theother.

Instead of a cathode rayV tube, a conventional light beam scanning device may be used to illuminate each of the unit cells.

The stored information is read from each cell by again scanning with the moving spot of light and applying the output voltage to a polarity sensing device such as that previously described. The -reversing switch is held at a .neutral position so that Ano current from the D.C. source is being applied to the cell.

Instead of applying and reading information from a multi-element screen by means of a flying spot scanning device, the apparatus ymaybe used by directing `a beam of energy at any particularelement of the screen desired without scanning 'the entire screen. Using a two-dimensional coordinate system, a focused light beam may be directed to one particular` element, or, a beam of electrons maybe 4directed to -a corresponding part of .a phosphor screen. vThis maybe kdone with suitable x, y 'coordinate controls for directing either the light bearnfor the electron .beam tothe desired pointon the screen. Conventional oscillographs khave controls for 'thus-directing the electron beam. The apparatus illustrated-in Figure-4 includesc'ontrols for applying -any desired relative Vpotentials to .each 'of the horizontal deflection .plates V52 and to each of the -vertical deflection .plates 54. These control means-may comprise .potentiometers 56 and 58 connected to a common source of D.C. potential. The sliding contact 60 of potentiometer 56 may be connected to one of the horizontal `deflection plates, the .other plate of the pair being grounded. '.In a similar manner, the sliding contact v62 of -potentiometer 58 is connected to one of the vertical deflection plates, kthe other being connected to ground.` By varying the settings of the sliding contacts in conventional manner, the potentials applied to the horizontal and vertical deflection plates may be varied so 'as ,to direct the beam of electrons to any desired part ofi the Iviewing screen40. There has thus been described a 'novel method ,and yapparatus for storing information of a binary nature and, later, reading the stored information. The invention is based on the discovery. that certain materials, such as :lead sulfide, telluride, or selenide, when specially activated,

exhibit a photo-voltaic effect with one polarity if current iszrst passed through in one directionduring light ,activation and the opposite polarity if current is first `passed lthrough in the :opposite direction while light is `directed onalm of the material.

:We claim as our invention:

l. Electrical :apparatus .comprising a cell having .fa

surface, a first portion of vwhich is .composedofaphotovconductive material and ,a second portion of 4which is composed 'of a photo-voltaic material capable ofvchang- .ing the direction of its 'photo-current output and Lconsisting essentially of directionally sensitive photovoltaic -lead sulfide in the form of a film -that can begiven '.a

first polarity-when current is passed therethroughin lone direction and Vcan be given the opposite polarity when current is passed therethrough in the opposite direction, said second portion being in -electrical contactvwith said first portion, and 'an electrode in contact with each of said portions.

2. A device accordingto claim l in which saidphotoconductive material is cadmium sulfide.

3. Electrical apparatus comprising a cell having abase plate, one portion of said plate being coated with a photoconductive material and another adjacent portion of 'which is coated with directionally sensitive photovoltaic lead sulfide in electrical contact with said photoconduc- 'tivematerial land in the form ofa film that can be given a first polarity when current is passed therethrough fin Eone ,direction and can be given the opposite polarity whencurrent Vis passed therethrough-in theopposite direction, and an electrode in contact with each of said portions.

4. Apparatus according to claim 3 including means for applying a potential of desired polarity to said electrodes.

5. Apparatus according to claim 4 including, in addition, means for applying a beam of light to both of said coated portions of said plate.

6. Apparatus according to claim V5 including, also, means connected to said electrodes for detecting the polarity of said electrodes.

7. Apparatus comprising a plate having a surface comprising a plurality of cellular elements, each of said elements comprising one surface portion film of a photoconductive material and an adjacent surface portion film of a directionally sensitive photo-voltaic material consisting essentially of lead sulfide in electrical contact with said photo-conductive material and that can be given a first polarity when current is passed therethrough in one direction and can be given the voppositelpolarity when current is passed therethrough in the opposite direction, an electrode connected to each of said portions of each of said elements, means for passing a weak current across said photovoltaic film in either of two opposite directions, means for directing a beam of light to any of said elements, and means for detecting the polarity of any of said electrodes when light is directed to the element connected thereto.

8. An information storage and indicating apparatus comprising an element including a film of material that can be made photo-voltaic with a first polarity when current is passed through said film in one direction and which can be made photo-voltaic with a second polarity opposite to that of said first polarity when current is passed through said film in another direction opposite to said one direction, a film of photo-conductive material adjacent and in contact with said film of photovoltaic material, and an electrode in contact with each of said films.

9. Apparatus according to claim 8 including means for applying a potential of desired polarity to said electrodes.

10. Apparatus according to claim 9 including means for applying a beam of light to said film.

11. Apparatus according to claim 10 including means for detecting the polarity of said electrodes when light is directed onto both of said lms.

12. A method of storing information of a binary nature comprising rendering directionally photo-voltaic in a desired sense a film of photo-voltaic material having the property of assuming a polarity in one sense when current is passed through in a first direction and of assuming a polarity in an opposite sense when current is passed through in a second direction opposite to said first direction, and, later, reading said information by directing a beam of light onto said film while detecting the polarity of said film.

13. A method according to claim 12 including placing a film of photo-conductive material adjacent and in contact with said film of photo-voltaic material and directing light onto said photo-conductive film whenever it is desired to pass a current through said photo-voltaic film.

14. Apparatus comprising a plate having a surface comprising a plurality of cellular elements, each of said elements comprising one surface portion film of a photoconductive material and an adjacent surface portion film of a directionally sensitive photovoltaic material in electrical contact with said photo-conductive material that can be given a first polarity when current is passed through said film in one direction and the opposite polarity when current is passed through in the opposite direction, an electrode connected to each of said portions of each of said elements, means for passing a weak current across said photovoltaic film in either of said two directions, means for directing a beam of light to any of said elements, and means for detecting the polarity of any of said electrodes when light is directed to the element connected thereto.

15. An electrical cell comprising a layer of photovoltaic material having the property of assuming a polarity in one sense when current is passed therethrough in a first direction and of assuming polarity in an opposite sense when current is passed therethrough in the opposite direction, means for selectively passing current through said layer in either of said directions and means for directing light upon said layer.

16. A cell according to claim 15 in which said photovoltaic material is lead sulfide.

References Cited in the le of this patent UNITED STATES PATENTS 2,160,383 Kannenberg May 30, 1939 2,258,294 Lubszynski et al. Oct. 7, 1941 2,406,139 Fink et al. Aug. 20, 1946 2,416,215 Rath Feb. 18, 1947 2,540,490 Bittner Feb. 6, 1951 2,698,928 Pulvari Ian. 4, 1955 

